I am kegging in a 5 gallon corny keg a root beer extract from Rainbow flavors. They give you 8 cups sugar to 4 gallons of water and 1 bottle extract. But the sugar is for yeast carbonation. I am going to CO2 carbonate and dispense from a tap on my kegerator for the kids.

Is 8 cups too much sugar? I am thinking more around the 6 cup.
Plus how many pounds is 6 cups? Because I am lazy